In lenticular imaging, an optical element called a lenticular screen is placed between an image and the viewer, as disclosed, for example, in U.S. Pat. Nos. 3,504,059 to Glenn, Jr.; 2,724,312 to Gruetzner; and 3,683,773 to Dudley. Lenticular screens are typically formed from a transparent plastic sheet with lenses integrally formed on only one side that focus on the opposite and typically flat side. The lenticular elements can have many shapes, such as, for example, cylindrical, angular, spherical, or shaped like cube corners.
The original lenticular image can be created photographically, electronically or by a combination of both. Images can be reproduced and mass produced using photo-sensitive materials, printed by lithographic means or by other means of printing or transfer of inks or dyes. Electronic displays, such as cathode-tubes or flat panel displays, may also be fitted with a lenticular screen if the resolution of the display is sufficient.
One known technique for manufacturing lenticular screens includes cutting the lenticular lens pattern directly into transparent plastic materials. A lenticular screen can also be formed by the casting of thermosetting resins onto flat forms including the lenticular lens pattern. In addition, a lenticular screen may also be made by thermoforming a cast or extruded transparent sheet by heat and pressure between one polished plate and one plate with the lenticular pattern cut or engraved into its surface. Injection molding may also be used to form a lenticular screen.
A lenticular screen may also be formed by extrusion of resin onto transparent pre-produced sheet or film, wherein the lenticular pattern is embossed into the resin by an embossing roll. The applied resin may then be cured by radiation through the transparent material while the web is in contact with the embossing roller. Alternately, if the properties of the resin permit, the resin may be cured by radiation after the sheet has left the embossing roll.
A lenticular screen may also be manufactured using a machine similar to that normally used for flat sheet extrusion. As disclosed in U.S. Pat. No. 2,724,312 to Gruetzner, for example, and as shown with reference to the prior art illustration of FIG. 1, such a machine 20 includes a sheet extruder 21 and a polishing stack 22. The polishing stack 22 illustratively includes three rolls 23, 24 and 25, wherein the middle roll 24 has the lenticular pattern on its outer surface. The upper roll 23 and lower roll 25 have a smooth outer surface. A plurality of small rolls 27 support the sheet 26, and a pair of pull rolls 28 pull the sheet from the polishing stack 22. This type of machine 20 and associated method are used for most of the lenticular sheet produced at present.
The lenticular image can be reproduced on the photographic material and thereafter be laminated to the lenticular screen. There are also applications where the photographic emulsion is coated onto the flat side of the screen. The lenticular image can also be printed with inks or dyes on a suitable material which, in turn, is laminated to the back of the screen or printed directly onto the flat side of the screen. Common for all the processes are that the lenticular image and the lenticular screen meet exacting tolerances to obtain the desired result.
The three roll polishing stack 22 is basically a simplified calendar. The polishing rolls 23, 24 and 25, are also known as chill rolls in flat plastic sheet production or as embossing rolls, if a pattern is cut or engraved into their surface. The functions of the rolls are to give the sheet, which initially is pre-formed by the sheet die at the end to the extruder 21, a more uniform gage, polish the sheet surface, transfer the pattern of the embossing roll to the sheet, and chill the material to a temperature lower than its softening point to keep the sheet in the shape formed by the rolls. The embodiment of the prior art machine 20' of FIG. 2 illustrates that the rolls 23', 24' and 25' can be arranged in a diagonal orientation. Of course a horizontal orientation is also possible.
Considering the arrangement of FIG. 1, if the three rolls in the stack 22 have one common motor drive, the upper roll 23 and middle roll 24 are held in position so they define a nip point therebetween. The lower roll 25 is held within a short distance to the middle roll 24, since the plastic is shrinking, and therefore the circumferential speed of the rolls needs to be differentiated. The plastic web 26 is taken away by the pull rolls 28 to keep the sheet in relative contact with the surface of the middle and lower rolls, but will also apply extra tension to the plastic web 26 as well. The tension caused by the pull rolls 28 will stretch the material.
In the case of individual drives for each of the rolls in the stack 22' shown in FIG. 2, the upper roll 23' can also form a nip with the middle roll 24'; however, its speed has to be slightly different to keep the plastic web 26' in contact with the middle roll. In addition, pull rolls, not shown, still have the task of keeping the plastic in contact with the upper roll 23'.
Unfortunately, in the prior art machines 20, 20' the sheet is held under pressure just in the nip between the first two rollers, and it is there that the lenticular lenses are created. The time the plastic sheet is under pressure in the nip will depend on the diameter of the rolls, the speed of the web and the thickness of the material. Even with larger diameter rolls, a thicker sheet, and at the speeds somewhat less than recommended to ensure proper flow through the extruder, the time period is relatively short. In fact, the time period is often too short to ensure proper reproduction of the lenticular pattern onto the plastic web. Downstream from this nip point, only the tensioning of the sheet enables contact of the sheet with the embossing roll. Or course, the contact is limited by the tension that can be applied to the still hot plastic either by the upper roll 23' (FIG. 2) or by the pull rolls 28 (FIG. 1).
The tensioning of the sheet can, for some materials that otherwise are suited for lenticular screens, cause a haze and/or make the sheet brittle. Excessive tensioning can result in stress that can be released even at temperatures lower than the softening point and cause the sheet to buckle. It is required, if the finished lenticular screen is to be optically acceptable, that the lenticular lenses in the plastic material replicate the pattern on the embossing roller as closely as possible.
In addition, the energy carried in the plastic sheet will have a significant negative effect on the valleys between adjacent lenses, and also on the lens curvature. Even with a relatively thin sheet, for example, having a thickness of about 0.4 mm, it can be difficult in a conventional polishing stack to obtain an accurate reproduction of the lens pattern. The thicker the sheet, the greater the problem with accurately reproducing the lenticular pattern. Some materials can, if cooled too quickly, leave a residue on the rolls, which if allowed to build up, would prevent the lenticular pattern from being accurately reproduced in the plastic. Accordingly, the machine is taken out of service and the residue removed, thereby resulting in a loss of production.
Yet another difficulty with conventional machines for making a lenticular screen is that production of the lenticular pattern forming roll itself is elaborate and costly. Lenticular screens can have a lens width, or pitch if the lens is cylindrical, smaller than 100 microns. One method of machining such fine and accurate lenses is by cutting the pattern into the outer surface of the roll with single crystal diamonds. The diamond tool can have a cutting edge which defines a radius or part of a sphere and which can be ground by conventional techniques or formed by chemical techniques. A common conventional technique for obtaining a hard lenticular pattern forming roll creates the pattern into the steel mantel of the roll and then applies a hard chrome plating for wear and corrosion resistance. Alternately, another material like copper is plated onto the mantel surface and the pattern is etched, engraved or cut in the copper, and thereafter the mantel is plated with hard chrome for wear resistance. The hard chrome surface itself can also be given a structure or pattern by etching or engraving.
Unfortunately, many of the conventionally used materials, such as steel, may cause considerable wear of the diamond cutting tool, and thereby increase the cost of forming the lenticular pattern. In addition, the entire lenticular pattern forming roll must be removed when worn, shipped to a remote site for refinishing, and transported back for re-installation. The roll is relatively bulky and, thus, costly to ship. In addition, the machine may be out of service while the roll is being refinished or replaced.